U.S. patent number 6,649,559 [Application Number 09/912,439] was granted by the patent office on 2003-11-18 for supported metal membrane, a process for its preparation and use.
This patent grant is currently assigned to dmc2 Degussa Metals Catalysts Cerdec AG. Invention is credited to Ernst Drost, Bernd Kempf, Werner Kuhn, Meike Roos, Stefan Wieland.
United States Patent |
6,649,559 |
Drost , et al. |
November 18, 2003 |
Supported metal membrane, a process for its preparation and use
Abstract
The invention provides a supported metal membrane which contains
a metal membrane on a support surface of a porous membrane support.
The supported metal membrane is obtainable by applying the metal
membrane to the support surface of the membrane support, wherein
the pores in the membrane support are sealed, at least in the
region of the support surface, prior to applying the metal membrane
and are opened by removing the auxiliary substance only after
applying the metal membrane.
Inventors: |
Drost; Ernst (Rannenbergring,
DE), Kuhn; Werner (Rodenbach, DE), Roos;
Meike (Biebergemund, DE), Wieland; Stefan
(Offenbach, DE), Kempf; Bernd (Kleinwallstadt,
DE) |
Assignee: |
dmc2 Degussa Metals Catalysts
Cerdec AG (Hanau, DE)
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Family
ID: |
7652326 |
Appl.
No.: |
09/912,439 |
Filed: |
July 26, 2001 |
Foreign Application Priority Data
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Aug 12, 2000 [DE] |
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100 39 596 |
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Current U.S.
Class: |
502/182; 428/116;
428/312.8; 428/319.1; 428/553; 502/439; 502/527.15; 96/4; 96/11;
95/56; 95/55; 502/527.24; 502/527.12; 428/613 |
Current CPC
Class: |
B01D
69/105 (20130101); B01D 53/228 (20130101); H01M
8/0687 (20130101); B01D 67/0072 (20130101); B01D
67/0069 (20130101); C01B 3/505 (20130101); B01D
71/022 (20130101); Y10T 428/12479 (20150115); Y02E
60/50 (20130101); B01D 2325/04 (20130101); C01B
2203/041 (20130101); C01B 2203/0475 (20130101); B01D
2325/10 (20130101); Y02P 70/50 (20151101); C01B
2203/0485 (20130101); Y10T 428/24149 (20150115); Y10T
428/24997 (20150401); Y10T 428/24999 (20150401); Y10T
428/12063 (20150115) |
Current International
Class: |
C01B
3/00 (20060101); B01D 53/22 (20060101); B01D
71/00 (20060101); B01D 71/02 (20060101); C01B
3/50 (20060101); B01J 021/18 (); B32B 003/00 ();
B01D 053/22 (); B01D 059/12 () |
Field of
Search: |
;502/182,527.12,527.15,527.24,439 ;428/319.1,312.8,116,613,553
;95/55.56 ;96/4,11 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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197 38 513 |
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Nov 1998 |
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DE |
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0 924 161 |
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Jun 1999 |
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EP |
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0 924 162 |
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Jun 1999 |
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EP |
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0 924 163 |
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Jun 1999 |
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EP |
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0 945 174 |
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Sep 1999 |
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EP |
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1208 962 |
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Oct 1970 |
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GB |
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1 292 025 |
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Oct 1972 |
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GB |
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WO 89/04556 |
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May 1989 |
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WO |
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WO 99/33545 |
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Jul 1999 |
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WO |
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Other References
Shigeyuki Uemiya et al., Hydrogen Permeable Palladium-Silver Alloy
Membrane Supported On Porous Ceramics, Journal of Membrane Science,
56, Mar. 1991, No. 3, Amsterdam, NL. .
T. S. Moss et al. "Composite Metal Membranes for Hydrogen
Separation Applications" Porc.-Natl. Hydrogen Assoc. Annu. U.S.
Hydrogen Meeting 8.sup.th (1997), pp. 357-365. .
T. S. Moss et al. "Multilayer Metal Membranes for Hydrogen
Separation" Int. J. Hydrogen Energy vol. 23, No. 2, (1998) pp.
99-106. .
WPIDS-140642 for JP 05078810 (Abstract) Mar. 1999. .
Y. Lin et al. "An Integrated Purification and Production of
Hydrogen With a Palladium Membrane-Catalytic Reactor" Catalysis
Today 44 (1998) pp. 343-349. .
Y. Lin et al. "Process Development For Generating High Purity
Hydrogen By Using Supported Palladium Membrane Reactor as Steam
Reformer as Steam Reformer" Int. J. Hydrogen Energy 25 (2000) pp.
211-219. .
E. Kikuchi "Membrane Reactor Application to Hydrogen Production"
Catalysis Today 56 (2000) pp. 97-101..
|
Primary Examiner: Bell; Mark L.
Assistant Examiner: Hailey; Patricia L.
Attorney, Agent or Firm: Kalow & Springut LLP
Claims
What is claimed is:
1. A supported metal membrane comprising a metal membrane on a
support surface of a porous membrane support, wherein the pores of
the membrane support are formed by electrochemically dissolving the
copper-rich phase from eutetic alloy AgCu.
2. The supported metal membrane of claim 1, wherein the metal
membrane consists of palladium or a palladium alloy.
3. The supported metal membrane of claim 2, wherein the palladium
alloy is PdAg23, PdCu40 or PdY10.
4. The supported metal membrane of claim 1, wherein the metal
membrane has a thickness of less than 5 .mu..
5. The supported metal membrane of claim 1, wherein the metal
membrane has a thickness of 2 to 0.3.mu..
6. The supported metal membrane of claim 1, wherein the metal
membrane is multilayered.
7. The supported metal membrane of claim 1, further comprising a
diffusion barrier layer between the metal membrane and the membrane
support.
8. The supported metal membrane of claim 1, wherein the supported
metal membrane is in the form of a foil or tubule.
9. The supported metal membrane of claim 1, wherein the membrane
support is in the form of a porous honeycomb.
10. The supported metal membrane of claim 1, further comprising a
catalytically active coating on the surface of the porous membrane
support opposite the metal membrane.
11. The supported metal membrane of claim 1, further comprising a
functional layer on the surface of the porous membrane support
opposite the metal membrane, wherein said functional layer is for
removing impurities and harmful substances.
12. A method for preparing a supported metal membrane comprising:
filling the pores of a porous membrane support with an auxiliary
substance in at least a region defining a support surface; then
applying a metal membrane to the support surface; and then removing
the auxiliary substance from the pores of the porous membrane
support.
13. The method of claim 12, further comprising smoothing and
cleaning the support surface after filling the pores of the porous
membrane support.
14. The method of claim 12, wherein the porous membrane support is
made from a porous metal, a metal alloy, a sintered metal, a
sintered steel, a glass or a ceramic, and the auxiliary substance
is a chemically readily removable metal, salt, graphite, polymer or
high molecular weight organic compound.
15. The method of claim 12, wherein the metal membrane is applied
by electrochemical deposition or by a PVD or a CVD process.
16. The method of claim 15, wherein the metal membrane is comprised
of palladium or a palladium alloy.
17. The method of claim 16, wherein the metal membrane contains
PdAg23, PdCu40 or PdY10.
18. A method for preparing a supported metal membrane comprising:
applying a metal membrane to a support surface of a membrane
support consisting of a multi-phase eutectic alloy having a phase
more base than another phase; then electrochemically dissolving the
more base phase from the membrane support to form pores in the
membrane support.
19. The method of claim 18, further comprising: cleaning the
support surface of the membrane support before applying the metal
membrane, and heating the applied metal membrane and membrane
support to a temperature between 300 and 700.degree. C. before
dissolving the more base phase from the membrane support.
20. The method of claim 18, wherein the multiphase eutectic alloy
is AgCu.
21. The method of claim 18, wherein the metal membrane is applied
by electrochemical deposition or by PVD or a CVD process.
22. The method of claim 21, wherein the metal membrane is comprised
of palladium or a palladium alloy.
23. The method of claim 22, wherein the metal membrane contains
PdAg23, PdCu40 or PdY10.
Description
DESCRIPTION
The invention provides a supported metal membrane which contains a
metal membrane on a porous membrane support, as well as a process
for its preparation and its use. Supported metal membranes of this
type are used for separating gas mixtures, in particular for the
separation of hydrogen from a reformate gas for supplying fuel
cells with the required fuel gas.
For this purpose, palladium or palladium alloy membranes on either
porous or non-porous supports are normally used, such as compact
palladium or palladium alloy membranes. Foils made of
hydrogen-permeable metals, inter alia, are used as non-porous
supports. The permeability of the membranes for hydrogen increases
with temperature. Typical operating temperatures are therefore
between 300 and 600.degree. C.
T. S. Moss and R. C. Dye [Proc.-Natl. Hydrogen Assoc. Annu. U.S.
Hydrogen Meet., 8th (1997), 357-365] and T. S. Moss, N. M. Peachey,
R. C. Snow and R. C. Dye [Int. J. Hydrogen Energy 23(2), (1998),
99-106 ISSN: 0360-3199] describe the preparation and use of a
membrane which is obtained by applying Pd or PdAg by cathode
sputtering (atomization) to both faces of a foil of a metal from
group 5B. The thickness of the layers applied to the two faces may
be varied so that an asymmetric component is produced (for example:
0.1 .mu.m Pd/40 .mu.m V/0.5 .mu.m Pd). Permeation trials
demonstrate twenty-fold higher hydrogen permeation as compared with
self-supported Pd membranes. Accordingly, the membrane described is
suitable for use in a PEM fuel cell system instead of the
traditional catalytic gas purification steps (water gas shift
reaction and preferential oxidation of CO).
GB 1 292 025 describes the use of iron, vanadium, tantalum, nickel,
niobium or alloys thereof as a non-porous support for a
non-coherent, or porous, palladium (alloy) layer. The palladium
layer is applied by a pressing, spraying or electrodeposition
process in a thickness of about 0.6 mm to a support with a
thickness of 12.7 mm. Then the thickness of the laminate produced
in this way is reduced to 0.04 to 0.01 mm by rolling.
According to DE 197 38 513 C1, particularly thin hydrogen
separation membranes (thickness of layer less than 20 .mu.m) can be
prepared by alternate electrodeposition of palladium and an alloy
metal from group 8 or 1B of the periodic system of elements to a
metallic support which is not specified in any more detail. To
convert the alternating layers into a homogeneous alloy,
appropriate thermal treatment may follow the electrodeposition
process.
Either metallic or ceramic materials are suitable as porous
supports for palladium (alloy) membranes. In accordance with JP
05078810 (WPIDS 1993-140642), palladium may be applied to a porous
support by a plasma spray process for example.
According to Y. Lin, G. Lee and M. Rei [Catal. Today 4.4 (1998)
343-349 and Int. J. of Hydrogen Energy 25 (2000) 211-219] a
defect-free palladium membrane (thickness of layer 20-25 .mu.m) can
be prepared on a tubular support made of porous stainless steel
316L in a electroless plating process and integrated as a component
in a steam reforming reactor. At working temperatures of 300 to
400.degree. C., a purified reformate containing 95 vol. % H.sub.2
is obtained. However, the optimum working temperature is very
restricted because below 300.degree. C. the palladium membrane
starts to become brittle due to the presence of hydrogen, whereas
above 400 to 450.degree. C. the alloying constituents in the
stainless steel support diffuse into the palladium layer and lead
to impairment of the permeation properties.
Electroless plating processes are preferably used for coating
ceramic supports. Thus, CVD coating of an asymmetric, porous
ceramic with palladium is described by E. Kikuchi [Catal. Today 56
(2000) 97-101] and this is used in a methane steam reforming
reactor for separating hydrogen from the reformate. The minimum
layer thickness is 4.5 .mu.m. If the layer is thinner, the
gas-tightness of the layer can no longer be guaranteed. Apart from
CVD coating with pure Pd, coating with palladium alloys is also
possible, wherein the alloy with silver prevents embrittlement of
the palladium membrane and increases the permeability to
hydrogen.
In addition to pure hydrogen separation membranes, membranes which
are provided with a reactive layer in addition to the hydrogen
separation layer (palladium) are also described for applications in
fuel cell systems. Thus, the porous support for a palladium (alloy)
membrane may be covered, for example on the face which is not
coated with Pd, with a combustion catalyst. The heat released
during combustion at the reactive face is then simultaneously used
to maintain the operating temperature of the hydrogen separation
membrane (EP 0924162 A1). Such a component may then be integrated
in the reforming process downstream of a reformer or incorporated
directly in the reformer (EP 0924161 A1, EP 0924163 A1).
In addition, not only palladium membranes can be used for hydrogen
separation in the fuel cell sector. EP 0945174 A1 discloses a
design for the use of universally constructed layered membranes
which may contain both fine-pore, separation-selective plastics
and/or several ceramic layers and/or layers made of a
separation-selective metal (preferably from groups 4B, 5B or 8),
wherein these layers are applied to a porous support (glass,
ceramic, expanded metal, carbon or porous plastics).
The objective of developing metal membranes for the separation of
hydrogen from gas mixtures is to obtain high rates of permeation
for the hydrogen. For this purpose, the metal membrane must be
designed to be as thin as possible while avoiding the occurrence of
leakiness in the form of holes. Such membranes can be processed
only in a supported form. In order for the membrane support to have
as little effect as possible on the permeation of hydrogen, it must
have a high porosity. Thus there is the difficulty, in the case of
known processes for preparing supported membranes, of depositing a
defect-free membrane on a porous support. There are two problems
involved here. On the one hand, the methods described for
depositing for example palladium or a palladium alloy can guarantee
a relatively defect-free membrane layer only above a certain
thickness of layer. This minimum thickness is about 4 to 5 .mu.m.
On the other hand, the coating techniques used for applying the
membrane layer to the porous membrane support means that the
average pore diameter of the membrane support ought not exceed a
certain value because otherwise it would be impossible to apply
coherent and defect-free coatings. The pore sizes of known membrane
support materials, such as porous ceramics or porous metal
supports, are therefore less than 0.1 .mu.m. This means that the
resistance to flow of the gas through the pores cannot be reduced
to a desirable extent.
WO 89/04556 describes an electrochemical process for preparing a
pore-free membrane based on palladium supported by a porous metal
structure. In accordance with the process, a pore-free
palladium(-silver) membrane on a porous, metallic support is
produced by coating one face of a metal alloy foil (preferably
brass) with palladium or palladium/silver (thickness of palladium
layer: about 1 .mu.m) using an electrodeposition process. The
porosity of the support is produced later by dissolving the base
components out of the brass foil. Dissolution is performed
electrochemically, wherein, in a cyclic process, both components
are first taken into solution but the more base component is
redeposited directly onto the palladium layer (electrochemical
recrystallisation). The less base component in the foil-shaped
alloy thus goes virtually quantitatively into solution so that a
porous metal structure, preferably a porous copper structure,
remains as a support for the palladium/silver membrane.
The process in accordance with WO 89/04556 has the disadvantage
that the brass foil used as support is virtually completely
dissolved and has to be built up again by electrochemical
recrystallisation. This means that the composite or laminate formed
between the palladium layer and the support foil is destroyed. The
mechanical strength of the recrystallised foil is low and its
porosity is undefined.
The object of the present invention is to provide a supported metal
membrane for the separation of hydrogen from gas mixtures which can
be prepared by a simple and cost-effective process. Another object
of the invention are supported metal membranes, in which the
membrane support has a hitherto unrealisable, high, porosity
(average pore sizes and pore volumes). A further object of the
present invention are composite metal membranes in which the
average pore size of the membrane support is greater than the
thickness of the metal membranes.
This object is achieved by a supported metal membrane which
contains a metal membrane on a support surface of a porous membrane
support. The supported metal membrane can be obtained by applying
the metal membrane to the support surface of the membrane support,
wherein the pores in the membrane support are sealed by an
auxiliary substance, at least in the area of the support surface,
prior to application of the metal membrane and are opened by
removing the auxiliary substance only after applying the metal
membrane.
In the context of the present invention, the support surface of the
membrane support and its contact surfaces are differentiated. The
support surface includes the entire surface area which is available
for coating with the metal membrane, that is the surfaces of the
pores sealed with auxiliary substance, which they have in the plane
of the support surface, and also the direct contact surfaces of the
membrane support with the metal membrane after removal of the
auxiliary substance.
The metal membrane according to the invention is obtainable, for
example, by choosing a porous membrane support in which the pores
are sealed with an auxiliary substance, either completely or only
in the region of the intended support surface. The membrane support
preferably consists of a porous metal, a metal alloy, a sintered
metal, a sintered steel, a glass or a ceramic. The pores in these
materials are sealed prior to application of the metal membrane by,
for example, a chemically readily removable metal, a salt,
graphite, a polymer or a high molecular weight organic
compound.
Before applying the metal membrane, it is recommended that the
support surface of the membrane support be smoothed by suitable
means such as grinding and polishing and in particular that the
subsequent contact surfaces with the metal membrane be exposed and
cleaned. The high surface quality produced in this way is
transferred to the metal membrane being applied and is retained
even after removing the auxiliary substance so that the final
supported metal membrane has a very flat structure with a uniform
layer thickness.
Depending on the properties of the auxiliary substance and the
membrane support, the auxiliary substance can be removed from the
pores of the membrane support in a variety of ways such as, for
example, by melting, burning out, dissolving, chemical dissolution
and electrochemical dissolution.
Electrochemical deposition or PVD or CVD processes are suitable for
applying the metal membrane to the membrane support. A preferred
PVD-process for depositing the metal membrane onto the membrane
support is cathode sputtering. This process generally results in
very dense layers with low porosity, i.e. with high packing
density.
The just described process for the preparation of a supported metal
membrane according to the invention includes the following steps:
a) filling the pores of the porous membrane support with the
auxiliary substance, b) smoothing and cleaning the support surface,
c) applying the metal membrane to the support surface and d)
removing the auxiliary substance from the pores of the membrane
support.
Another possibility for preparing a supported membrane comprises
choosing an initially non-porous membrane support which has a
potential porosity. The term "potential porosity" indicates that
the membrane support has an inhomogeneous structure, wherein the
subsequent pores are filled by an auxiliary substance which is
removed only after applying the metal membrane to the support
surface of the membrane support.
This can be achieved in a simple manner when the membrane support
consists of a multi-phase eutectic alloy and the auxiliary
substance is formed by the more base (more electronegative) phase
arranged in phase domains and this is electrochemically dissolved
with the production of pores after application of the metal
membrane. The eutectic alloy AgCu which consists of a Cu-rich and
an Ag-rich alloy phase is especially suitable for this purpose. The
Cu-rich phase is more electronegative and can be selectively
dissolved out of the membrane support with the production of the
desired porosity using an electrochemical route. The Ag-rich phase
then remains almost untouched. Whereas, in accordance with WO
89/04556, the membrane support is completely dissolved and rebuilt,
according to the present invention a rigid framework of the Ag-rich
alloy phase is retained, with corresponding positive effects on the
stability of the membrane support.
The copper content of the eutectic alloy is preferably between 20
and 80 wt. %, with respect to the total weight of alloy. By
suitable thermal treatment of the support at 400 to 750.degree. C.,
before or after applying the metal membrane, its overall structure,
and thus its subsequent porosity, can be affected in an
advantageous manner.
To summarise: the process for preparation of a supported metal
membrane according to the invention using a membrane support made
from an eutectic alloy as described above comprises the following
process steps: a) cleaning the support surface of the membrane
support, b) applying the metal membrane to the support surface, c)
treating the laminate of metal membrane and membrane support at
temperatures between 300 and 700.degree. C. and d)
electrochemically dissolving the more base phase in the membrane
support.
The supported metal membrane according to the invention is
preferably used as a gas separation membrane for the separation of
hydrogen from gas mixtures. In this case, the metal membrane is
preferably prepared from palladium or a palladium alloy, for
example from PdAg23, PdCu40 or PdY10.
A small thickness of metal membrane is required for use as a gas
separation membrane in order to ensure the highest possible
permeability for the desired gas. Gas separation membranes of
palladium or palladium alloys with a thickness of more than 20
.mu.m are of only small interest for the separation of hydrogen
from gas mixtures due to the high cost of the noble metal and the
low permeability. Membranes with a thickness of less than 0.3 .mu.m
may have a number of defects. In addition, the permeability for
undesired gases also increases at these small thicknesses. As a
result of these two effects, the separating power of a membrane
with a membrane thickness of less than 0.3 .mu.m drops to values
which are no longer tolerable. Therefore the metal membrane
preferably has a thickness between 0.3 and 5, preferably between
0.5 and 3 .mu.m.
The porous metallic membrane support is used to support the thin
metal membrane, wherein the membrane support should impair the
permeability of the metal membrane as little as possible, as
compared with a freely suspended metal membrane of the same
thickness. On the other hand, a certain minimum thickness is
required in order to ensure requisite mechanical stability of the
supported membrane. The thickness of the membrane support should
therefore be less than 100 .mu.m and should not be less than 20
.mu.m. Thicknesses of the membrane support between 0.50 and 20
.mu.m are preferably striven for.
When using the supported metal membrane as a gas separation
membrane for hydrogen containing gas mixtures it has to withstand
strongly varying operating conditions with time. This leads to
temporal changes of membrane volume and dimensions as a result of
incorporation and release of hydrogen and temperature changes.
Changes in dimension of the membrane should be comparable to those
of the membrane support to avoid disruption of the supported metal
membrane. Therefore, metal composite membranes (metal membrane on a
metallic membrane support) are preferred over heterogeneous
metal-ceramic-composites (metal membrane on a ceramic support) when
changes to volume or dimensions due to temperature changes are a
problem. The thermal expansion coefficients of two metals exhibit
less differences than the expansion coefficients of a metal and a
ceramic.
From the above mentioned membrane materials PdAg23, PdCu40 and
PdY10 the alloy PdAg23 is subject to considerably stronger changes
in dimension and volume due to hydrogen incorporation than the
alloy PdCu40. Therefore, a metal membrane made from PdCu40 on a
membrane support based on AgCu is the preferred metal composite
membrane for purifying hydrogen.
It is often an advantage to build up the metal membrane as a
multilayered structure. In this case, it is possible to design the
first layer, lying directly on the membrane support, as a diffusion
barrier. The diffusion barrier should prevent, in particular for
metallic membrane supports, any change in alloy composition in the
metal membrane due to diffusion of alloy constituents into the
membrane or out of the membrane taking place when using the
supported metal membrane. A change in alloy composition of this
type may have a considerable effect on the permeability of the
metal membrane. Ceramic oxides such as, for example, aluminium
oxide, titania and ceria are suitable as diffusion barriers. As an
alternative to diffusion barriers from oxidic materials metal
layers from vanadium, tantalum or niobium can be employed. These
metals have a good permeability for hydrogen. The thickness of
these diffusion barrier layers should be less than 0.5 .mu.m in the
case of oxide layers and less than 2 .mu.m in the case of a metal
barrier. Preferably the thickness of the barrier layer is less than
0.1 .mu.m in both cases.
When using the supported metal membrane to purify reformate gas, it
may be expedient to combine the supported metal membrane with a
catalyst. For this purpose, a catalytically active coating is
applied to the surface of the porous membrane support opposite to
the metal membrane. Alternatively, a functional layer to remove
impurities and harmful substances may be applied instead of the
catalytically active coating.
The supported membrane according to the invention is preferably
used for the separation of hydrogen from gas mixtures, in
particular from reformate gases. The invention enables the
preparation of supported metal membranes in which the membrane
supports have a hitherto unrealisable, high, porosity (average pore
sizes and pore volumes). With thicknesses of gas separation
membrane of 0.3 to 5, preferably 0.5 to 3 .mu.m, the membrane
support has an average pore size greater than 0.5 and less than 10
.mu.m. Thus, for the first time a supported metal membrane is
described here in which the average pore size of the membrane
support is greater than the thickness of the metal membrane. It
therefore has outstanding hydrogen permeability.
In general, the supported metal membrane will be used in the form
of plane foils. But the metal membrane can also be produced in the
form of varying geometrical structures which have the additional
advantage of improved mechanical stability compared to plane foils
of the same thickness. In particular, the supported metal membrane
can be manufactured in the form of thin tubules.
The invention is explained in more detail by means of FIGS. 1 to 6
and the following examples:
FIG. 1: idealised cross section of a supported metal membrane
according to the invention before the auxiliary substance is
removed from the pores of the membrane support
FIG. 2: idealised cross section of a supported metal membrane
according to the invention after the auxiliary substance has been
removed from the pores of the membrane support
FIG. 3: idealised cross section of a supported metal membrane
according to the invention with a diffusion barrier layer between
metal membrane and membrane support
FIG. 4: idealised cross section of a supported metal membrane
according to the invention with a diffusion barrier layer between
metal membrane and membrane support and with a catalytic coating on
the surface of the membrane support opposite to the metal
membrane
FIG. 5: cross section of an experimental PdAg-membrane on a
AgCu-membrane support taken with a raster electron microscope
FIG. 6: porous structure of a membrane support consisting of an
eutectic AgCu-alloy after dissolution of the Cu-rich phase
FIG. 1 shows an idealised illustration of a cross section of a
supported metal membrane according to the invention before the
auxiliary substance is removed from the pores of the membrane
support. Reference numeral (1) denotes the composite metal
membrane, i.e. the composite comprising the metal membrane (2) and
the membrane support (3). The surface area of the membrane support
at the interface between the metal membrane and the membrane
support is the formerly defined support surface (4). The support
surface is composed of different surface areas which comprise areas
(7) formed by the membrane support material (5) and areas (8)
formed by the pores (6) filled with the auxiliary substance in the
plane of the support surface (4). The areas (8) have been defined
as contact surfaces beforehand.
FIG. 2 shows the same cross section as in FIG. 1 after removing of
the auxiliary substance from the pores of the membrane support.
During operation of the supported metal membrane as gas separation
membrane for cleaning of hydrogen, material of the membrane support
may diffuse into the metal membrane (2) and lead to unintentional
reduction of the hydrogen permeability of the metal membrane. For
lowering this diffusion a diffusion inhibiting barrier (9) can be
introduced between metal membrane (2) and membrane support (3).
FIG. 3 shows a cross section of a composite membrane according to
this invention with such a diffusion barrier between membrane and
support. Suitable materials for the diffusion barrier are alumina,
titania and ceria and metal layers made from vanadium, tantalum or
niobium as already mentioned above.
FIG. 4 shows an embodiment of the supported metal membrane
according to the invention having a functional layer (10) deposited
onto the surface of the membrane support opposite to the metal
membrane. The functional layer may be a catalytic layer for
converting carbon monoxide by water gas shift reaction, a layer for
oxidising carbon monoxide to carbon dioxide or the functional layer
may be an absorbing layer for absorbing sulphur components such as
hydrogen sulphide.
EXAMPLE 1
Thin Pd layers with layer thicknesses of 0.1, 0.5 and 2 .mu.m were
prepared on foils of AgCu28 by electrodeposition. The AgCu28 foil
had a thickness of 50 .mu.m.
After thermal treatment of the coated foils under a protective gas
(argon) at 600.degree. C. for a period of 30 min, the Cu-rich phase
was dissolved out of the AgCu28 alloy material in the membrane
support. Dissolution was performed anodically in a sulfuric acid
electrolyte using 10% strength sulfuric acid operated
potentiostatically at 40.degree. C. and with a constant bath
voltage of 230 mV over the course of 20 hours. This produced an
open-pore structure in the membrane support foil.
Metallographic examination and images produced by a scanning
electron microscope over the cross-section of the finally produced
supported metal membrane showed a firmly adhering, dense Pd
membrane on a porous AgCu support layer with open porosity and a
pore size of 1 to 5 .mu.m.
EXAMPLE 2
Using a PdAg23 target a PdAg23 layer, 2 .mu.m in thickness, was
deposited onto a foil of AgCu28 by cathode sputtering.
After thermal treatment of the coated foil under a protective gas
(argon) at 600.degree. C. for a period of 30 min, the Cu-rich phase
was dissolved out of the AgCu28 alloy material in the membrane
support. Dissolution was performed anodically in a sulfuric acid
electrolyte using 10% strength sulfuric acid operated
potentiostatically at 40.degree. C. and with a constant bath
voltage of 230 mV over the course of 20 hours. This produced an
open-pore structure in the membrane support foil.
FIG. 5 shows the cross section of the thus produced metal composite
membrane taken with a scanning electron microscope after
dissolution of the Cu-rich phase of the membrane support. From FIG.
5 the large pore structure of the membrane support can clearly be
seen. The average pore size is larger than the thickness of the
metal membrane. The metal membrane has a flatness which had not
been achievable if the metal membrane had been deposited onto a
porous membrane support. As shown in FIG. 5, the average pore
diameter increases with increasing distance from the metal membrane
and is largest at the surface of the membrane support opposite to
the metal membrane. This gradient pore structure is due to the
anodic dissolution of the Cu-rich phase of the membrane support
described above.
EXAMPLE 3
A further membrane support foil of AgCu28 was used to investigate
the influence of thermal treatment on the formation of the pore
structure. The Cu-rich phase was dissolved out of the foil as
described in examples 1 and 2.
FIG. 6 shows a cross section of the membrane support foil after
dissolution of the Cu-rich phase. The foil had been subjected to a
different thermal treatment than the foils in the preceding
examples. The average pore diameter of the pore structure is much
smaller than in FIG. 5 and indicates that the porosity and its
structure can be influenced by the thermal treatment of the
eutectic membrane support during production of the membrane support
foil.
Thermomechanical forming of the AgCu28 alloy during rolling to
obtain the desired foil thickness and the secondary thermal
treatment determine the pore structure of the final membrane
support. Rapid cooling of the AgCu28 alloy during production leads
to small phase regions and results in small average pore diameters
after dissolution of the Cu-rich phase. Extended secondary thermal
treatment after thermomechanical forming initiates
re-crystallisation of the eutectic alloy and thus leads to an
increase in size of the phase regions and to large average pore
sizes of the completed membrane support as demonstrated in example
2. In addition, the size of the phase regions can be influenced by
changing the overall composition of the alloy.
Though in the foregoing examples only membrane supports based on an
eutectic AgCu28 alloy have been used, the invention is not
restricted to the use of such eutectic alloys as membrane support
materials. As already mentioned before, porous membrane supports
can be used of which the pores have been filled with an auxiliary
substance before deposition of the metal membrane and only after
the application of the metal membrane is the auxiliary substance
removed from the pores.
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